RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
25 
felt as an inconvenience ; so that by a very simple contrivance, founded on the prin- 
ciple of counterbalancing every part of the apparatus, and employing the clock 
only to regulate and controul the action of a driving weight, and not to do the work 
of putting the apparatus in motion, I was enabled with very little trouble to adapt 
such a movement to an axis of good workmanship which had formerly served as the 
support of a Bird’s quadrant. This being fixed in the proper position, carried round, 
diurnally, a frame on which were attached, with every necessary adjustment, the 
Fraunhofer prism, the achromatic lens and the focal screen above mentioned, as well 
as any other occasional and needful apparatus for varying the experiments, especially 
for interposing coloured glasses, and plate-glass rectangular boxes, destined for the 
reception of coloured and other liquids, whose absorptive action on the chemical rays 
it should be desired to try. 
68. In my first experiments with an apparatus thus disposed, I employed a large 
prism of plate glass filled with water, for the sake of operating on a large beam of 
light. But however large the refracting angle, the dispersion of water proved too 
low, the spectrum being too short, in proportion to the sun’s angular diameter, to 
give distinct and sharply defined results. It was therefore disused, and the Fraun- 
hofer flint prism substituted: and here I may take occasion to mention, that whether 
water, crown, or flint glass was used, I was not sensible of any peculiarity of action 
(apart from difference of dispersive power) which I could fairly attribute to the qua- 
lities of those media. I confess I had looked for a contrary result, and had specu- 
lated, with somewhat excited expectation, on the discovery of bands, or interruptions, 
or points of maximum and minimum action in the spectrum, traceable to unequal 
absorption of the chemical rays by those media, though colourless. None such how- 
ever appeared. But on this point I would hardly be understood to speak, as yet, de- 
cisively in the negative. 
69. One of the first uses to which this apparatus was applied was an endeavour to 
ascertain the existence or nonexistence of fixed lines in the chemical spectrum ana- 
logous to those in the luminous, and whether corresponding, or not, in place. To 
this end it was necessary to diminish the sun’s angular apparent diameter, which was 
done by affixing a small achromatic telescope with a low magnifying power, in a re- 
versed position (eye-piece towards the sun), to receive and refract the rays before 
their incidence on the prism. By this means, spectra were obtained whose lengths 
were in a highly increased proportion to their breadths. In one experiment this pro- 
portion (as ascertained by measurement of the impression left) was so high as 67 : 1 . 
But in this, as in Other experiments where a less degree of dilatation was obtained, 
the interruptions, if any, to the continuity of chemical action from end to end, were 
so equivocal that I cannot venture to assert their existence positively, though I am 
led to believe it from the fact of some slight inequalities in the violet region having 
been repeated, in the same place of the spectrum, on shifting the paper to eliminate 
the possible influence of streaks left by the brush in its preparation. The only cer- 
mdcccxl. e 
